Wastewater treatment method for removing organic matter and nitrogen, carrier used thereof and method for manufacturing the carrier

ABSTRACT

A wastewater treatment method for removing organic matter and nitrogen from wastewater. In the wastewater treatment method, an excessive amount of organic matter which prevents nitrifying bacteria from growing is removed in a leading aeration tank, and the conditions of the aeration tank are optimized using the nitrifying carrier to which a great amount of nitrifying bacteria can be attached for growth, such that the nitrifying bacteria can multiply therein. As a result, a large amount of wastewater can be treated within a short time, and it is possible to stably cope with a change in load of the organic matter of the wastewater. Thus, the wastewater treatment apparatus can be smaller, and an improvement in performance thereof can be expected. Also, the wastewater can be treated stably during the winter season when the activity of the nitrifying bacteria becomes low, so that the wastewater treatment method according to the present invention can be applied to most wastewater treatment plants, e.g., sewage treatment plants, excrement treatment plants, livestock and industrial wastewater treatment plants and the like.

TECHNICAL FIELD

The present invention relates to a method for treating wastewatercontaining organic matter and nitrogen, and more particularly, to awastewater treatment method utilizing a carrier capable of enhancing theremoval efficiency of organic matter and nitrogen.

BACKGROUND ART

An active sludge method, a basic wastewater treatment method, has beenwidely used in the secondary treatment of wastewater after it hasundergone a first treatment, or in order to completely treat theoriginal wastewater in aerobic conditions. In a general active sludgemethod, as the wastewater continuously flows into an aeration tank,microorganisms grow by intaking the organic matter of the wastewatersuch that the organic matter is decomposed, and the grown microorganismscoagulate to settle in a terminal settling tank. A portion of theprecipitant returns to the aeration tank in the form of an activesludge, and the remaining portion is discarded as waste sludge, suchthat the amount of microorganisms in the aeration tank is maintained ata suitable level and the nitrogen and phosphorus are removed as theorganic matter is decomposed.

Such an active sludge method has been acknowledged as an effectivewastewater treatment method. However, this method is not suitable foreffectively removing the organic matter and nitrogen at the same timefrom wastewater containing a high-concentration of organic matter andnitrogen, and increases the surplus sludge. In particular, whenwastewater contains a high loading ratio of organic matter, bulkingoccurs in the settling tank, thereby lowering treatment efficiency. Inaddition, heterotrophic bacteria grow to excess due to the high loadingratio of the organic matter while growth of autotrophic bacteria whichinherently grow slowly is suppressed, so that active nitrification isnot possible.

To solve the above problems, a biofilm process has been developed inwhich a large number of microorganisms fixed to a carrier are used.According to the biological membrane process, it is necessary tomaintain the number of microorganisms in an aeration tank, used forwastewater treatment, to a suitable level. To this end, amicroorganism-attachable carrier is essential for suspended-growth orattached-growth of the microorganisms. Carrier for the purpose includeporous plastics(presented by Sung-yong Choi, in Journal of KoreanSociety on Water Quality, Vol. 6, No. 1, page 31, 1990), active carbonfiber (Japanese Patent Application No. Heisei 5-167820), polyvinylalcohol and active carbon (Japanese Patent Application No. Heisei5-186723) or the like. The material of the carrier is low in hydrophilicproperties, so that it is not easy to attach microorganisms to thecarrier. Also, the microorganisms not attached to the carrier is low ingrowth rate, so that the microorganisms flows out of the aeration tankwhen the wastewater treatment tank is continuously operated.

Also, in the conventional biofilm process utilizing a fixed typecarrier, because a biofilm is formed on the surface of the carrier inexcess, the amount of dissolved oxygen (DO) supplied to nitrifyingbacteria whose growth rate is relatively slow is not enough, so that itis hard to maintain the amount of the nitrifying bacterial to a suitablelevel. In addition, if the conventional biofilm membrane processutilizes a single carrier, and there is a limitation in theconcentration of nitrogen removed.

Meanwhile, ammonia nitrogen of the wastewater is treated in two stagesof nitrification and denitrification by a biological treatment method.That is, during nitrification, the ammonia nitrogen is changed intonitrate nitrogen (NO₃—N) by aerobic nitrifying bacteria, and duringdenitrification, denitrifying bacteria oxidize the organic matter usingthe nitrate nitrogen as an electron acceptor, instead of oxygen theamount of which is insufficient, and reduce the nitrate nitrogen tonitrogen (N₂). However, the activity of the nitrifying bacteria whichare sensitive to a temperature decrease lowers during the winter season,thus the nitrogen removal efficiency is also rapidly lowered.

DISCLOSURE OF THE INVENTION

To solve the above problems, it is an object of the present invention toprovide a method for stably and effectively treating wastewatercontaining a high-concentration of organic matter and nitrogen, by whichthe above-mentioned problems of the conventional biofilm process, suchas bulking, biofilm slough-off or decrease in treatment efficiencyduring the winter season, can be solved.

According to an aspect of the present invention, there is provided awastewater treatment method comprising the steps of: (a) denitrifyingwastewater inflowed into an anoxic tank by reducing the nitrate nitrogeninto gaseous nitrogen using denitrifying bacteria being resident in theanoxic tank, and organic matter of the inflowed wastewater; (b) makingthe carbon-to-nitrogen (C/N) ratio of the wastewater having undergonethe step (a) suitable for a subsequent nitrification step afterinflowing the wastewater into a first aeration tank by decomposing anexcessive amount of organic matter using aerobic microorganisms attachedto a carrier; (c) changing ammonia nitrogen of the wastewater havingundergone the step (b) into nitrate nitrogen after inflowing thewastewater into a second aeration tank by using nitrifying bacteriaattached to a carrier; (d) returning part of the wastewater havingundergone the step (c) to the anoxic tank, and inflowing the remainingwastewater to a settling tank; and (e) returning part of the sludgedischarged from the settling tank to the anoxic tank, discarding theremaining sludge as surplus sludge, and obtaining a supernatantseparated from the sludge settled in the settling tank as the treatedwater.

Preferably, after the organic matter is decomposed by the aerobicmicroorganisms in the step (b), the resulting wastewater is directlysent to the step (d) if the load of ammonia nitrogen of the wastewaterto be treated is small, thus omitting the step (c). Preferably, thewastewater passed through the step (a) is directly sent to the step (c)if the load by the organic matter of the wastewater to be treated issmall, omitting the step (b).

Preferably, the carrier used in the step (b) comprises a foamed polymer,powdered active carbon attached to the foamed polymer, and an adhesivefor sticking the powdered active carbon to the foamed polymer.

Preferably, the foamed polymer is polyurethane, polystyrene orpolyethylene, having a sponge foam or non-woven fabric structure.

Preferably, the adhesive is a mixture of acrylic resin andstyrene/butadiene Latex (S/B Latex) in a weight ratio of 70:30˜90:10.

Preferably, the powdered active carbon and the adhesive exist in aweight ratio of 40:60˜50:50.

According to another aspect of the present invention, there is provideda method for preparing the carrier, comprising the steps of: (a) forminga slurry by mixing an adhesive solution and a powdered active carbon ina weight ratio of 90:10˜70:30; (b) coating the slurry on a foamedpolymer, and evaporating solvent from the adhesive solution by heatingthe resulting product; and (c) molding the resulting product of the step(b).

Preferably, the heating of the step (b) is performed at 70˜95° C. for2˜3 hours.

Preferably, the carrier used in the step (c) is a polyvinyl alcohol foamcarrier or a cellulose fiber carrier.

Preferably, the cellulose carrier is prepared by the steps of: (a)molding a cellulose fiber into a planar shape; (b) infiltrating theplanar cellulose fiber with a foaming composition containing polyvinylalcohol, a cross- linking agent and a foaming agent; (c) foaming theresulting product of the step (b) by dehydrating; (d) infiltrating theresulting product of the step (c) with a reinforcing compositioncontaining polyvinyl alcohol and a cross-linking agent; and (e) dryingthe resulting product of the step (d) taken out of the composition.

Preferably, the foaming composition comprises 0.5˜5 wt % of polyvinylalcohol, 0.2˜5 wt % of cross-linking agent, 0.1˜1 wt % of foaming agent,and solvent as the remainder.

Preferably, the reinforcing composition comprises 1˜5 wt % of polyvinylalcohol, 0.1˜5 wt % of cross-linking agent, and solvent as theremainder.

Preferably, the cross-linking agent is melamine urea resin or polyamidepolyamine epichlorhydrine (PPE).

Preferably, dehydration of the step (c) is performed such that the watercontent of the planar cellulose fiber before foaming is maintained at20˜50% based on the weight of the planar cellulose fiber.

Preferably, foaming in the step (c) is performed at 120˜150° C. suchthat the volume of pores per unit gram of the foamed cellulose carrieron the dry basis is in the range of 0.3˜4.5 cm³.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram illustrating a wastewater treatment methodaccording to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIG. 1, wastewater 1 containing an organic matter andnitrogen flows into an anoxic tank 10. In the anoxic tank 10, oxidationof the organic matter and reduction of nitrate nitrogen (NO₃—N) occursconcurrently. That is, the organic matter of the influent wastewater isoxidized acting as an electron donor during denitrification, and thenitrate nitrogen contained in the influent wastewater and in returnwater 5 from a second aeration tank 30 accepts electrons to be reducedinto nitrogen (N₂), thereby removing the organic matter and nitrogenfrom the wastewater.

However, in the case where the wastewater containing a large amount oforganic matter flows into the anoxic tank 10, an excess of organicmatter compared to the nitrate nitrogen exists, so that untreatedorganic matter may exist in the effluent water 2 from the anoxic tank10. When a high concentration of organic matter exists, microbial florachanges in the second aeration tank 30, so that growth of filamentousfungi is facilitated. Here, if the filamentous fungi grow to excess, aserious problem may occur during separation of supernatant from asettling tank 40.

Thus, in the present invention a first aeration tank 20 containing acarrier for removing the organic matter is interposed between the anoxictank 10 and the second aeration tank 30 such that the excess organicmatter which could not be treated in the aeration tank 10 is removed.Because the carrier for removing the organic matter serves as a habitatfor an excess amount of microorganisms such that the microorganismsstuck to the carrier absorb a high concentration of organic matter, theexcessive growth of the filamentous fungi can be suppressed effectively.

In general, powdered active carbon has good absorbing capacity andresistance to a shock load caused by a sudden influx of wastewatercontaining a large amount of organic matter, and is capable ofprocessing wastewater stably even when the wastewater contains a toxicsubstance, so that powdered active carbon has been widely used toprocess wastewater. However, the powdered active carbon put in theaeration tank flows out of the aeration tank together with the sludge,so that the powdered active carbon must be supplied continuously intothe aeration tank to be kept at a predetermined concentration. Also,when the sludge is returned, the powered active carbon existing in thereturn sludge causes trouble to a pump. In addition, in order toseparate the powered active carbon from the waste sludge for the purposeof regeneration, an extra separator and regenerator for the powderedactive carbon is required.

Thus, a carrier for removing organic matter, which is suitable for thewastewater treatment method according to the present invention, is madefrom a foamed polymer, powdered active carbon attached to the foamedpolymer, and an adhesive for sticking the powdered active carbon to thefoamed polymer.

The foamed polymer is in a sponge foam or non-woven fabric structure.Preferably, the foamed polymer is formed of polyurethane, polystyrene orpolyethylene. The adhesive may be formed of a mixture of acrylic resinand styrene/butadiene Latex(S/B Latex) in a weight ratio of 70:30˜90:10.

Preferably, the powdered active carbon and the adhesive exist in aweight ratio of 40:60˜50:50.

Preferably, the carrier for removing the organic matter suitable for thewastewater treatment method according to the present invention ismanufactured by the following method.

First, the powdered active carbon used in a general wastewater treatmentmethod is added to an adhesive solution containing acrylic resin and S/BLatex and stirred, resulting in a slurry.

It is preferable that the powdered active carbon has a smaller particlesize for providing sufficient porosity. For example, the size of thepowdered active carbon may be 200 mesh or less. Also, it is preferablethat the adhesive solution and the powdered active carbon are mixed in aweight ratio of 90:10˜70:30, thereby resulting in a carrier containingthe adhesive and the powdered active carbon in a weight ratio of60:40˜50:50 after removing the solvent. Here, if the content of thepowdered active carbon is more than the above range, the bindingefficiency with the adhesive is decreased. On the other hand, if thecontent of the powdered active carbon is less than the above range, theadsorptivity of the powdered active carbon is lowered. That is, when thepowdered active carbon and the adhesive solution are mixed in the aboveweight ratio, the resulting carrier can have a predetermined level ofstrength, abrasion resistance and adsorptivity. And even though theresulting carrier is used for a long time, separation of the powderedactive carbon does not occur. Thus, the carrier can be used for severalyears.

Then, the slurry is coated on the foamed polymer. Preferably, the slurryis coated on the foamed polymer using a roller. Here, it is preferableto evenly coat the foamed polymer such that the slurry penetrates intothe inner pores of the foamed polymer. However, any general coatingmethod may be used.

The foamed polymer coated with the slurry is heated at 70˜95° C. for 2˜3hours to remove the solvent. During the heating process, curing reactionoccurs between the powdered active carbon and the adhesive.

Then, the resulting material is cut to a proper size during the moldingprocess. Here, the carrier is not limited to a specific shape. However,preferably, the carrier is processed into a hexahedral shape having alength, a width and a height all within the range of 0.8˜2.0 cm forconvenience in the molding process.

The amount of carrier for removing the organic matter put into the firstaeration tank 20 is controlled to be 5˜25% with respect to the volume ofthe tank. If the amount of the carrier is less than the above range, thevolume of the carrier to which the microorganism is attachable is nothigh enough. If the amount of the carrier added is more than the aboverange, smooth circulation of the carrier within the first aeration tank20 is difficult due to the excessive volume of the carrier.

An effluent water 3 from the first aeration tank 20, whosecarbon-to-nitrogen (C/N) ratio is controlled suitably, flows into asecond aeration tank 30 containing a carrier for nitrification toundergo nitrification. The condition of the second aeration tank 30 iscontrolled to be suitable for the activity of nitrifying bacteriaremoving ammonia nitrogen of the wastewater. That is, because of the lowconcentration of the organic matter, the nitrifying bacteria whosegrowth rate is slow relative to other microorganisms can grow muchfaster in the carrier.

A nitrifying carrier for fixing the nitrifying bacteria must be capableof fixing a large amount of microorganisms and keeping the attachment ofthe microorganism. For this purpose, the nitrifying carrier must have ahigh hydrophilicity considering properties of nitrifying bacteria.

However, the nitrifying carrier in use cannot easily absorb themicroorganism due to its low hydrophilicity and the microorganismattached to the carrier is also easily detached. Also, most nitrifyingcarriers have a planar structure (Boler, The Water Science andTechnology, Vol. 22(1), pp89, 1990), thus the microorganism attachablesurface area per unit volume is small. As a result, a large amount ofmicroorganisms cannot be attached to the planar carrier. Also, theattached microorganism cannot be protected from shear stress applied byfluid due to its planar structure, thus the attached microorganism iseasily separated.

Thus, preferably, a nitrifying carrier suitable for the wastewatertreatment method according to the present invention is polyvinyl alcoholfoam carrier or cellulose carrier which has a high hydrophilicity and alarge surface area per unit volume due to its high porosity.

The polyvinyl alcohol foam carrier contains a large amount ofhydrophilic hydroxyl(—OH) group, thus it is easily hydrated, and hence,easily adsorbs the nitrifying bacteria. Also, the surface area per unitvolume is large due to the foamed structure having a large number ofpores, and the durability thereof is good. Thus, the manufacture of anitrifying carrier by cutting a polyvinyl alcohol foam to a proper sizeand shape increases the efficiency.

Also, the cellulose carrier is manufactured using a hydrophiliccellulose fiber obtained from natural material, e.g., cotton, wood pulp,ramie, hemp, jute and flax, so that it has a high affinity to thenitrifying bacteria. The cellulose carrier is manufactured by thefollowing method.

That is, the method for manufacturing the cellulose carrier includes thesteps of: (a) molding a cellulose fiber into a planar shape; (b)infiltrating the planar cellulose fiber with a foaming compositioncontaining polyvinyl alcohol, a cross-linking agent and a foaming agent;(c) foaming the resulting product of the step (b) by dehydrating; (d)infiltrating the resulting product of the step (c) with a reinforcingcomposition containing polyvinyl alcohol and a cross-linking agent; and(e) drying the resulting product of the step (d) taken out of thecomposition.

In detail, the cellulose fiber having a length within the range of 0.5˜2mm is pre-formed into a planar shape. This is because it makes thepost-treatment easy without the need for dispersing the cellulose fiberin a solvent. The molding method is not limited to a specific method,and a similar method to that of paper making may be used. For example,after preparing a fiber slurry by dispersing the cellulose fiber inwater, the prepared fiber slurry is poured into a frame having apredetermined shape (e.g., a hexahedron shape). Then, a predeterminedpressure is applied to the fiber slurry in the frame for dehydration,resulting in a cellulose fiber molded into a planar shape.

Then, the cellulose fiber molded into a planar shape is infiltrated withthe foaming composition, and then foamed to provide a suitable conditionfor the growth of microorganisms adsorbed into the carrier. The foamingcomposition contains polyvinyl alcohol of 0.5˜5 wt %, a cross-linkingagent of 0.1˜5 wt %, a foaming agent of 0.1˜1 wt %, and solvent as theremainder. Here, water is preferable as the solvent. Also, any foamingagent capable of generating an excessive amount of nitrogen or carbondioxide through reaction with the solvent may be used withoutlimitation. In particular, a water-soluble foaming agent is preferred.Preferably, sodium bicarbonate is used as the foaming agent. Also,melamine urea resin or polyamide polyamine epichlorhydrine (PPE) can beused as the cross-linking agent.

For ensuring sufficient foam, the water content of the cellulose fibermolded into a planar shape must be controlled to be within the range of20˜50 wt % of the total weight of the cellulose fiber molded into aplanar shape. If the water content is less than the above range, thecellulose fiber molded into a planar shape may be deformed by abruptfoaming. If the water content exceeds the above range, the foaming timeis extended. Preferably, the foaming step is performed at 120˜150° C.such that the volume of pores per unit gram of the foamed cellulosecarrier on the dry basis belongs to the range of 0.3˜4.5cm³.

After the foaming step, the foamed cellulose carrier may be shrunkbecause it does not have a high enough rigidity. Thus, the foamedcellulose carrier is treated with a reinforcing composition in order tostabilize the foamed structure. The reinforcing composition contains apolyvinyl alcohol of 1˜5 wt %, a cross-linking agent of 0.1˜5 wt % andsolvent as the remainder. Here, this cross-linking agent contained inthe reinforcing composition is the same as that for the foamingcomposition, and water is preferable as the solvent. If theconcentration of the reinforcing composition exceeds the above range,the excessive amount of components of the composition acts as acontaminant on the surface of the carrier. Also, it is preferable torepeat the treatment with the reinforcing agent such that the finalcontent of the polyvinyl alcohol and the final content of thecross-liking agent become 5˜15 wt % and 3˜8 wt %, respectively, withrespect to the weight of the cellulose fiber.

The reinforcing composition increases the strength of the carrier bychemical binding the polyvinyl alcohol and the cross-linking agentcontained therein with cellulose fiber. Also, unreacted cross-linkingagent (PPE) has a positive charge under neutral or alkaline conditions,thus the surface of the carrier has a positive charge. Accordingly, mostnitrifying bacteria having a negative charge is easily attached to thesurface of the carrier. Also, once the nitrifying bacteria is attachedto the surface of the carrier, the nitrifying bacteria is hardlyseparated from the carrier.

Preferably, the resulting product after treatment with the reinforcingcomposition is dried quickly at about 100° C. such that the reinforcingcomposition cannot move on the surface of the carrier.

After the drying step, the resulting product may be cut to a proper sizeand shape. Here, the size and shape of the carrier as a final productare not limited.

The amount of the nitrifying carrier put into the second aeration tank30 is controlled to be 5˜30% with respect to the volume of the tank 30.If the amount of the nitrifying carrier added is less than the aboverange, the volume of the carrier to which the microorganism isattachable cannot reach the suitable level. If the amount of the carrieradded is more than the above range, smooth circulation of the carrierwithin the second aeration tank 30 is difficult due to the excessivevolume of the carrier. Thus, it is difficult to ensure efficientnitrification within the tank 30.

The large amount of nitrifying bacteria grown being attached to thecarrier can change ammonia nitrogen of the wastewater into nitratenitrogen through an aerobic reaction. In particular, the methodaccording to the present invention shows a very high nitrogen removalefficiency during the winter season a time during which the activity ofthe nitrifying bacteria becomes slow, compared to an active sludgemethod in which microorganisms are suspended, due to the highconcentration of nitrifying bacteria attached to the carrier.

Part of the effluent water 4 from the second aeration tank 30 isreturned to the anoxic tank 10 for denitrification, as a return water 5containing nitrate nitrogen. Also, the remaining effluent water 4 istransferred to the settling tank 40. Part of settled sludge 6 isreturned to the anoxic tank 10, as a return sludge 7, and the remainingsludge is discarded as surplus sludge 8. Thus, a supernatant withoutcontaining the sludge is obtained as treated water 9.

Also, it is known to those skilled in the art that a perforated platefor preventing overflow of the carrier, or an airlift pump forpreventing shifting of the carrier toward an outlet is installed in theaeration tank into which the carrier is put.

In the wastewater treatment method according to the present invention,an excessive amount of organic matter which prevents the growth of thenitrifying bacteria is removed in the leading aeration tank, and theoptimal condition where the nitrifying bacteria can multiply is preparedusing the nitrifying carrier in which a large amount of attachednitrifying bacterial is grown. As a result, the problem of bulking ofthe sludge, which was one of serious problems in the conventional activesludge method, can be solved. Also, a large amount of wastewater can betreated within a short time, and the wastewater treatment methodaccording to the present invention can cope with a change in load of theorganic matter in the wastewater, thus the wastewater treatmentapparatus can be minimized and provides high performance. Also, thenitrogen removal efficiency can be stably maintained even during thewinter season when the nitrogen removal efficiency is lowered by adecrease in the activity of the microorganisms.

Until now, the case where loads of the organic matter and ammonianitrogen of the wastewater are both high has been described. However, inthe case where only the load by the organic matter is low, thewastewater may be treated through only nitrification while omitting theorganic matter removing step. Also, in the case where the load of theammonia nitrogen of the wastewater is low, the wastewater may be treatedthrough only the organic matter removing step without the nitrificationstep.

Hereinafter, the present invention will be described through thefollowing examples. However, the present invention is not limited to thefollowing examples.

Carrier For Removing Organic Matter

Preparation Example 1

Firstly, 160 g of acrylic resin emulsion (N-140, Aurora Chemical Co.),which is equivalent to 64 g of acrylic resin on a solid basis, 40 g ofstyrene/butadiene Latex (KSL 106, S/B Latex, Kumho Petrochemical Co.),which is equivalent to 16 g of S/B Latex on a solid basis, werecompletely mixed, and 60 g of powdered active carbon (Union Co.,particle size of 200 mesh) was then added to the mixture, resulting in aslurry. The prepared slurry was coated several times on polyurethanehaving a sponge structure using a roller, and then heated at 75° C. forabout 2 hours. The resulting structure was cooled down to roomtemperature, and cut to make a hexahedral shape having a length, a widthand a height all of approximately 1 cm, resulting in a carrier accordingto the present invention.

EXAMPLE 1

The carrier prepared above was put into an aeration tank of a wastewatertreatment apparatus used for treating general organic wastewater,constructed of a first settling tank, the aeration tank and a secondsettling tank, the carrier taking up 20% of the total volume of theaeration tank.

Then, wastewater was flowed into the wastewater treatment apparatus.Here, the chemical oxygen demand (COD) of the wastewater allowed to flowin was 1000 mg/l and the total retention time was set to 6 hours. Then,the COD of the water which flowed out of the wastewater treatmentapparatus was measured by Standard method and COD removal ratio wascalculated. The results are tabulated in Table 1. Also, whetherseparation of powered active carbon and bulking occur during continuousoperation for 1 week was investigated, and the degree of the separationof microorganisms was also investigated.

Comparative Example 1

Wastewater was treated under the same conditions as in Example 1, exceptthat an active sludge method without using a carrier was used. Theresult is tabulated in Table 1.

Comparative Example 2

Wastewater was treated under the same conditions as in Example 1, exceptthat a conventional fixed type carrier, Saran Lock (Kureha Chemical Co.,Japan), was put into the aeration tank. The result is tabulated in Table1.

Comparative Example 3

Wastewater was treated under the same conditions as in Example 1, exceptthat a conventional suspended biomass carrier, Linpor (Linde Co.,Germany), was put into the aeration tank. The result is tabulated inTable 1.

TABLE 1 COD removal occurrence separation of degree of ratio of powderedactive separation of examples (%) bulking carbon microorganism Example 195 No barely occur barely occur Comparative 12 Yes powdered activeslight Example 1 carbon not used Comparative 89 No powdered activemoderate Example 2 carbon not used Comparative 62 Yes powdered activeslight Example 3 carbon not used

As can be understood from Table 1, the COD removal ratios when theconventional carriers were used were very high compared to the case ofusing a general active sludge method, but was still low compared to thecase of using the carrier according to the present invention. Also, theseparation of the powdered active carbon and microorganisms barelyoccurred, and bulking did not occur.

Adsorptivity Test

The adsorptivity of the carrier according to the present invention wasmeasured to determine whether the adsorptivity of powdered active carbonwas maintained after being coated on a foamed polymer as in the presentinvention.

An adsorptivity test was performed on the carrier prepared in thePreparation Example 1 using the same wastewater treatment apparatus usedas in Example 1. Here, phenol concentration of the influent wastewatermeasured by a standard method was 450 mg/l. After 2 hours, the phenolconcentration of the treated wastewater decreased to 45 mg/l, thusindicating a 90% phenol removal ratio.

As can be understood from the result, the carrier according to thepresent invention, prepared by coating the powdered active carbon on thefoamed polymer, still has a high adsorptivity capable of adsorbingchemicals existing in the wastewater as well as serving as a habitat formicroorganisms. Thus, the carrier according to the present invention canbe efficient even when wastewater flows in in which the load of organicmater largely varies.

Cellulose Nitrifying Carrier

Preparation Examples 2-6

A cellulose fiber having a length of about 0.5˜2 mm was dispersed inwater, resulting in slurry. The slurry was poured into a hexahedralframe having a plurality of small holes in the bottom, and apredetermined pressure was applied to the slurry to remove the water,resulting in a cellulose fiber molded into a planar shape. 5 g ofpolyvinyl alcohol (saponification value 90%, degree of polymerization1700) and 5 g of PPE were added to 50 g of water, and 0.5 g of sodiumbicarbonate, a foaming agent, was added to the mixture, and then stirreduntil completely mixed, resulting in a foaming composition. Then, thecellulose fiber molded into a planar shape was infiltrated with thefoaming composition such that the foaming composition was sufficientlyabsorbed into the molded fiber. Then, the resulting product wasdehydrated using a dehydrator such that the water content of theresulting product was 30% with respect to the weight of the cellulosefiber, and then the dehydrated product was foamed in an oven whosetemperature was set to 120˜150° C. The resulting foamed body wasinfiltrated with a reinforcing composition having the composition shownin Table 2, and then stirred for 10 minutes or more such that thereinforcing composition was evenly distributed on the foamed body. Next,the resulting foamed body was squeezed to remove the excessive amount ofreinforcing agent from the surface, and put in an oven whose temperaturewas set to 100° C. Then, the resulting product was dried for a short ofabout 5 minutes to prevent the reinforcing composition from moving onthe surface of the foamed body, resulting in a cellulose nitrifyingcarrier.

To investigate the effect of the reinforcing composition on the tensilestrength of the cellulose nitrifying carrier, the tensile strengths ofthe nitrifying carriers prepared in the Preparation Examples 2-5 treatedwith a reinforcing composition and the nitrifying carrier of thePreparation Example 6 not treated with a reinforcing composition weremeasured by a standard method, and the results are shown in Table 2.

TABLE 2 reinforcing composition polyvinyl alcohol tensile strengthexample (wt %) PPE (wt %) (g/cm²) Preparation 1 0.1 800 Example 2Preparation 3 0.3 1066 Example 3 Preparation 4 0.4 1150 Example 4Preparation 5 0.5 1200 Example 5 Preparation 0 0.0 400 Example 6

As can be understood from Table 2, the tensile strength of the carrierincreases greatly by treatment with the reinforcing composition. Also,the higher the contents of polyvinyl alcohol and PPE of the reinforcingcomposition are, the more the tensile strength of the carrier isenhanced. Thus, the cellulose nitrifying carrier according to thepresent invention can be used for a long time due to its high tensilestrength, and has the merit of being a natural material.

Preparation Examples 7-11

The carriers were prepared by the same method as in the PreparationExamples 2-5, except that the contents of polyvinyl alcohol and PPE ofthe reinforcing composition were varied as shown in Table 3. The chargeamount at the surface of each carrier was measured, and the results areshown in Table 3.

TABLE 3 reinforcing composition positive charge polyvinyl alcohol amountexample (wt %) PPE (wt %) (meg/g) Preparation 1 0.0 0 Example 7Preparation 2 0.2 1.2 Example 8 Preparation 3 0.6 2.6 Example 9Preparation 4 1.2 3.1 Example 10 Preparation 5 2.0 4.5 Example 11

As can be understood from Table 3, the positive charge amount could notbe measured at the surface in the case where PPE was not added. Also,the more the contents of added PPE were, the higher the positive chargeamount at the surface was. The reason why the amount of positive chargeincreases by treatment with PPE appears to be that unreacted nitrogenatoms of PPE have positive charges. Thus, the amount of positive chargeat the surface increases as the PPE content of the reinforcingcomposition increases. The increase of the positive charge amount at thesurface rapidly adsorbs nitrifying bacteria having a negative charge andslow-growing microorganisms. As a result, the time required for initialstabilization of the wastewater treatment apparatus is reduced, thus thetotal operation time thereof is shortened.

EXAMPLE 2 and Comparative Example 2

In order to measure the wastewater treatment efficiency of the cellulosenitrifying carrier according to the present invention, the wastewatercontaining ammonia nitrogen was inflowed into a wastewater treatmentapparatus with a varying ammonia concentration. Then, the ammonianitrogen contents of the wastewater treated with the carrier prepared bythe Preparation Example 4 (Example 2) and of the wastewater treated byan activated sludge process (Comparative Example 2) were measured byStandard method, the ammonia nitrogen removal ratios were calculated,and the results are shown in Table 4. Here, the total retention time ofthe wastewater in the wastewater treatment apparatus was 6 hours.

TABLE 4 ammonia concentration of influent wastewater ammonia nitrogenremoval ratio (%) (mg/l) Example 2 Comparative Example 2 50 99 or more80 100 99 or more 50 200 95 20 300 90 10

As can be understood from Table 4, in the case of using the cellulosenitrifying carrier according to the present invention, the ammonianitrogen removal efficiency was higher compared to the case of using theactive sludge method. Also, the efficiency of removing the ammonianitrogen by the carrier according to the present invention barelychanged even when the ammonia concentration of the influent wastewaterincreased. Thus, the cellulose nitrifying carrier according to thepresent invention can be stably used for organic wastewater in which theload of the organic matter is high.

EXAMPLE 3

The wastewater was treated using both a carrier for removing organicmatter and a nitrifying carrier. In a wastewater treatment apparatusused, the volumetric ratio of an anoxic tank, a first aeration tank anda second aeration tank was 1:1:1, and the organic matter removingcarrier of the Preparation Example 1 was put in the first aeration tankand the cellulose nitrifying carrier of the Preparation Example 10, wasput in the second aeration tank, with a 10% volumetric ratiorespectively.

Then, the influent wastewater was treated at room temperature (25° C.).Here, the biological oxygen demand (BOD) of the influent wastewater was120 mg/l and total Kjeldahl nitrogen (TKN) thereof was 60 mg/l. Bychanging the retention time from 8 hours to 3 hours, i.e., by graduallyincreasing the inflowing amount of wastewater, the BOD and TKN of theoutflow wastewater were measured by Standard method, and BOD removalratio and TKN removal ratio were calculated. The results are shown inTable 5.

EXAMPLE 4

The wastewater was treated by the same method as in Example 3, exceptthat the organic matter removing carrier of the Preparation Example 1and the cellulose nitrifying carrier of the Preparation Example 4 wereused.

Comparative Example 3

The organic wastewater was treated by the MLE (Modified LudzackEttinger) method utilizing a general suspended microorganism, under thesame conditions as in Example 3, and the results are shown in Table 5.Here, each volume of the aeration tank (two aeration tanks were used)was equal to the volume of the first and second aeration tanks inExample 3, respectively.

As shown in Table 5, the BOD removal ratio and the TKN removal ratiodecreased as the retention time became shorter. However, the degree ofthe decrease in BOD removal ratio and TKN removal ratio were smaller inthe case of using the method according to the present invention than inthe case of using the conventional method. The reason for maintainingsuch a high removal ratio even when the amount of influent wastewaterincreases is that the carrier is capable of adsorbing a large amount ofmicroorganisms. The concentration of the microorganisms within theaeration tank was 2000 mg/l for suspended microorganisms and 2500 mg/lfor microorganisms attached as a biofilm, i.e., a total of 4500 mg/l inExamples 3 and 4, and only a total of 2000 mg/l in the ComparativeExample 1.

TABLE 5 retention BOD removal ratio (%) TKN removal ratio (%) timeExample Example Comparative Example Example Comparative (hrs) 3 4Example 3 3 4 Example 3 8 95 95 95 99 99 95 6 95 93 93 99 99 90 4 93 8585 95 96 75 3 92 70 70 95 96 50

EXAMPLES 5-6

For the purpose of comparing the wastewater treatment efficiency duringthe winter season, the wastewater was treated in the same manner as inExamples 3 and 4, except that the retention time was set to 6 hours, andthe temperature of the wastewater was changed to 25° C., 20° C., 15° C.and 8° C.

The BOD and TKN of the effluent water passed through the wastewatertreatment apparatus were measured by Standard method, and BOD and TKNremoval ratios according to the temperature decrease of the wastewaterwere calculated. The results are shown in Table 6.

Comparative Example 4

The wastewater was treated under the same conditions as in Example 5,using the MLE process used in Comparative Example 3, and the results areshown in Table 6.

TABLE 6 tempera- BOD removal ratio (%) TKN removal ratio (%) tureExample Example Comparative Example Example Comparative (° C.) 5 6Example 4 5 6 Example 4 25 95 95 95 99 99 95 20 94 95 90 97 99 93 15 9392 84 95 96 80  8 90 90 80 90 91 50

As shown in Table 6, in the wastewater treatment method according to thepresent invention, the high TKN removal ratio was maintained and the BODremoval ratio was enhanced at a low temperature. This is because theorganic matter was removed in the leading aeration tank, and theconditions suitable for the attached-growth of the nitrifying bacteriawere created using the nitrifying carrier.

INDUSTRIAL APPLICABILITY

As described above, in the wastewater treatment method according to thepresent invention, an excessive amount of organic matter which preventsthe nitrifying bacteria from growing is removed in the leading aerationtank, and the condition of the aeration tank is optimized using thenitrifying carrier to which a great amount of nitrifying bacteria can beattached for growth, such that the nitrifying bacteria can multiplytherein. As a result, a large amount of wastewater can be treated withina short time, and it is possible to stably cope with a change in load ofthe organic matter of the wastewater. Thus, the wastewater treatmentapparatus can be smaller, and an improvement in performance thereof canbe expected. Also, the wastewater can be treated stably during thewinter season when the activity of the nitrifying bacteria becomes low,so that the wastewater treatment method according to the presentinvention can be applied to most wastewater treatment plants, e.g.,sewage treatment plants, excrement treatment plants, livestock andindustrial wastewater treatment plants and the like.

What is claimed is:
 1. A wastewater treatment method comprising thesteps of: (a) denitrifying wastewater flowing into an anoxic tank byreducing the nitrate nitrogen into gaseous nitrogen using denitrifyingbacteria being resident in the anoxic tank, and organic matter of thewastewater flowing into the anoxic tank; (b) making thecarbon-to-nitrogen (C/N) ratio of the wastewater having undergone thestep (a) suitable for a subsequent nitrification step after thewastewater flows into a first aeration tank by decomposing an excessiveamount of organic matter using aerobic microorganisms attached to acarrier; (c) changing ammonia nitrogen of the wastewater havingundergone the step (b) into nitrate nitrogen after the wastewater flowsinto a second aeration tank by using nitrifying bacteria attached to acarrier; (d) returning part of the wastewater having undergone the step(c) to the anoxic tank, while the remaining wastewater flows into asettling tank; and (e) returning part of the sludge discharged from thesettling tank to the anoxic tank, discarding the remaining sludge assurplus sludge, and obtaining a supernatant separated from the sludgesettled in the settling tank as the treated water.
 2. The method ofclaim 1, wherein the carrier used in the step (b) comprises: a foamedpolymer; powdered active carbon attached to the foamed polymer; and anadhesive for sticking the powdered active carbon to the foamed polymer.3. The method of claim 2, wherein the foamed polymer is polyurethane,polystyrene or polyethylene, having a sponge foam or non-woven fabricstructure.
 4. The method of claim 2, wherein the adhesive is a mixtureof acrylic resin and styrene/butadiene Latex (S/B Latex) in a weightratio of 70:30˜90:10.
 5. The method of claim 2, wherein the powderedactive carbon and the adhesive exist in a weight ratio of 40:60˜50:50.6. A method of claim 2, further comprising the step of preparing thecarrier used in step (b) of claim 1, said carrier preparing stepcomprising the steps of: (a) forming a slurry by mixing an adhesivesolution and a powdered active carbon in a weight ratio of 90:10˜70:30;(b) coating the slurry on a foamed polymer, and evaporating solvent fromthe adhesive solution by heating the resulting product; and (c) moldingthe resulting product of the step (b).
 7. The method of claim 6, whereinheating of the step (b) is performed at 70˜95° C. for 2˜3 hours.
 8. Themethod of claim 1, wherein said carrier used in the step (c) comprises apolyvinyl alcohol foam carrier or a cellulose fiber carrier.
 9. Themethod of claim 8, wherein the cellulose fiber is obtained from cotton,wood pulp, ramie, hemp, jute or flax.
 10. The method of claim 8, furthercomprising the step of preparing the cellulose carrier by the steps of:(a) molding a cellulose fiber into a planar shape; (b) infiltrating theplanar cellulose fiber with a foaming composition containing polyvinylalcohol, a cross-linking agent and a foaming agent; (c) foaming theresulting product of the step (b) by dehydrating; (d) infiltrating theresulting product of the step (c) with a reinforcing compositioncontaining polyvinyl alcohol and a cross-linking agent; and (e) dryingthe resulting product of the step (d) taken out of the composition. 11.The method of claim 10, wherein the foaming composition comprises 0.5˜5wt % of polyvinyl alcohol, 0.2˜5 wt % of cross-linking agent, 0.1˜1 wt %of foaming agent, and solvent as the remainder.
 12. The method of claim11, wherein the cross-linking agent is melamine urea resin or polyamidepolyamine epichlorhydrine (PPE).
 13. The method of claim 10, wherein thereinforcing composition comprises 1˜5 wt % of polyvinyl alcohol, 0.1˜5wt % of cross-linking agent, and solvent as the remainder.
 14. Themethod of claim 13, wherein the cross-linking agent is melamine urearesin or polyamide polyamine epichlorhydrine (PPE).
 15. The method ofclaim 10, wherein dehydration of the step (c) is performed such that thewater content of the planar cellulose fiber before foaming is maintainedat 20˜50% based on the weight of the planar cellulose fiber.
 16. Themethod of claim 10, wherein foaming in the step (c) is performed at120˜150° C. such that the volume of pores per unit gram of the foamedcellulose carrier on the dry basis is in the range of 0.3˜4.5cm³.
 17. Awastewater treatment method comprising the steps of: (a) denitrifyingwastewater flowing into an anoxic tank by reducing the nitrate nitrogeninto gaseous nitrogen using denitrifying bacteria being resident in theanoxic tank, and organic matter of the wastewater flowing into theanoxic tank; (b) making the carbon-to-nitrogen (C/N) ratio of thewastewater having undergone the step (a) suitable for a subsequentnitrification step after the wastewater flows into a first aeration tankby decomposing an excessive amount of organic matter using aerobicmicroorganisms attached to a carrier; (c) changing ammonia nitrogen ofthe wastewater having undergone the step (b) into nitrate nitrogen afterthe wastewater flows into a second aeration tank by using nitrifyingbacteria attached to a carrier if the load of ammonia nitrogen of thewastewater to be treated exceeds a predetermined level; (d) returningpart of the wastewater having undergone at least step (b) to the anoxictank, while the remaining wastewater flows into a settling tank; and (e)returning part of the sludge discharged from the settling tank to theanoxic tank, discarding the remaining sludge as surplus sludge, andobtaining a supernatant separated from the sludge settled in thesettling tank as the treated water.
 18. A wastewater treatment methodcomprising the steps of: (a) denitrifying wastewater flowing into ananoxic tank by reducing the nitrate nitrogen into gaseous nitrogen usingdenitrifying bacteria being resident in the anoxic tank, and organicmatter of the wastewater flowing into the anoxic tank; (b) making thecarbon-to-nitrogen (C/N) ratio of the wastewater having undergone thestep (a) suitable for a subsequent nitrification step after thewastewater flows into a first aeration tank by decomposing an excessiveamount of organic matter using aerobic microorganisms attached to acarrier if the load by the organic matter of the wastewater to betreated exceeds a predetermined level; (c) changing ammonia nitrogen ofthe wastewater having undergone at least step (a) into nitrate nitrogenafter the wastewater flows into a second aeration tank by usingnitrifying bacteria attached to a carrier; (d) returning part of thewastewater having undergone the step (c) to the anoxic tank, while theremaining wastewater flows into a settling tank; and (e) returning partof the sludge discharged from the settling tank to the anoxic tank,discarding the remaining sludge as surplus sludge, and obtaining asupernatant separated from the sludge settled in the settling tank asthe treated water.